Medical apparatus with cable guide

ABSTRACT

A cable guide for X-ray scanners includes an elastomer-coated guide roller which rolls between concentric, parallel drums one end of the cable being fastened to one drum and passing around the roller.

The invention relates to a cable guide which comprises a cylindricalinner drum which is rotatable relative to a cylindrical outer drum and arotatable cylindrical cable guide roller which is disposed between theconcentrically disposed drums, the cylinder axis of said roller beingparallel to the cylinder axes of the drums, around which roller aflexible cable is passed which near one of its ends is attached to oneof the drums.

The term "cylindrical drum" is to be understood in the broadest sense.As will appear from the following description said term is assumed toinclude also sectors of drums i.e. rotationally asymmetrical bodieswhich have a similar function. Generally speaking, the term curved guidepath might be used.

A cable guide of the type mentioned in the preamble is for examplesuitable for use in apparatus for X-ray diagnostics and X-ray therapy,for guiding cables which interconnect components which are rotatablerelative to each other, the magnitude of the relative rotation generallyranging from a fraction of one revolution to some full revolutions. Thecable may comprise both flexible electrical cores as well as flexiblecores for conveying gases and liquids.

From British Patent Specification No. 1,127,191 a cable guide of thetype mentioned in the preamble is known, in which a plurality ofrotatable cable guide rollers are mounted on a carrier which in itselfis rotatable about an axis which coincides with the drum axes. When theinner drum and the outer drum are rotated relative to each other, thecarrier is driven with the aid of a gear transmission which comprisessix slip couplings and six ratchet-wheel couplings. Owing to a specificslippage in the coupling the flexible cable--which is attached both tothe inner drum and to the outer drum--is always kept taut duringrotation of the drums relative to each other. In the space between thetwo drums the flexible cable, depending on the relative direction ofrotation, is wound from the inner drum onto the cable guide rollers orfrom the cable guide rollers onto the inner drum, without becomingentangled.

A drawback of the known cable guide described hereinbefore is thecomparatively large number of moving parts.

It is an object of the invention to provide a simple cable guide with aminimum of moving parts which are consequently subject to wear.

A cable guide in accordance with the invention is thereforecharacterized in that the cable guide roller is adapted to roll betweenthe outer surface of the inner drum and the inner surface of the outerdrum over geometrical paths which are concentric with the inner drum andthe outer drum.

During rotation of the inner drum relative to the outer drum, the cableguide roller thus rolls over the outer surface of the inner drum andover the inner surface of the outer drum, the rolling distances of thecable guide roller measured along the outer surface of the inner drumand the inner surface of the outer drum being equal, the cable which ispassed around the cable guide roller rolls from the inner drum to theouter drum or from the outer drum to the inner drum, depending on thedirection of rotation relative to each other.

A preferred embodiment of a cable guide in accordance with the inventionis characterized in that the outer surface of the cable guide roller isat least partly covered with a layer of an elastic material. Preferablythis layer consists of a foam plastic. By lining the cable guide rollerwith an elastic layer manufacturing tolerances in respect of the outerdiameter of the inner drum, the inner diameter of the outer drum and thediameter of the cable guide roller may be comparatively large.

A cable guide in accordance with the invention is particularly suitablefor compact incorporation into a medical apparatus with a radiationsource which is rotatable relative to a frame, which source is connectedto an electrical power supply by means of a flexible cable, so that theinvention also relates to medical apparatus comprising such a cableguide.

A particular medical apparatus of this type is characterized in that amovable frame is suspended to the inner drum, in which frame a detectoris secured opposite the radiation source mounted in the frame, thecentral ray of the radiation source intersecting the cylinder axes ofthe two drums and being parallel to the direction of movement of theframe, whilst the two drums are constituted by sectors of circularcylinders, which have a free area in which the radiation source ismovable when the frame is moved relative to the inner drum.

The invention will now be described in more detail with reference to thedrawing, which shows an example of a specific application of the cableguide. In the drawing:

FIG. 1 is a longitudinal section of a medical apparatus for computertomography in accordance with the invention, provided with a cable guidein accordance with the invention,

FIG. 2 is a cross-section taken on the line II--II of the apparatusshown in FIG. 1,

FIG. 3 shows a flexible multi-core cable which is particularly suitablefor use in the cable guide in accordance with FIGS. 1 and 2, and

FIG. 4 schematically illustrates a special effect, which a cable guidein accordance with the invention is found to exhibit.

FIG. 1 and FIG. 2 in longitudinal section and cross-section respectivelyshow a medical apparatus for computer tomography, where a patient 1which is positioned on an examination table 2 is exposed to a planarfan-shaped X-ray beam 3 which is emitted by an X-ray source 4. Theradiation transmitted by the patient is measured with an X-ray detector5, which comprises a series of detection elements 6 which are arrangedon an arc of a circle. The X-ray source 4 and the X-ray detector 5 aresimultaneously rotated about an axis 7, which is normal to the X-raybeam 3 during examination. With the aid of the computer the densitydistribution of the patient in the exposed planar slice is computed fromthe information supplied by the X-ray detector 5. An accuratecalculation is possible if the X-ray source 4 and X-ray detector 5 arerotated through an angle of approximately 400°. For this purpose theX-ray source 4 and the X-ray detector 5 are connected to a ring 8 whichis journalled on rollers 10 which are arranged in a frame 9 and which isrotatable about the axis 7 with the aid of a drive motor 11.

The X-ray source 4 and the X-ray detector 5 are connected to equipment13--which inter alia includes a high-voltage supply for the X-raysource--by means of a flexible multicore cable 12, which is accommodatedin the frame 9 by a cable guide 14. The cable guide 4 comprises acylindrical inner drum 15 which is secured to the ring 8, a cylindricalouter drum 16 which is secured to the frame 9, and a cylindrical cableguide roller 17 which is retained between the inner and the outer drum.The inner drum 15 and the outer drum 16 are disposed concentrically andtheir cylinder axes coincide with the axis 7. The cylinder axis of thecable guide roller 17 is parallel to the axis 7. The cable 12--which ispassed around the cable guide roller 17--is attached to the inner drum15 near an end 18 and to the outer drum 16 near an end 19. During arelative rotation of the inner and outer drum the centre of the cableguide roller moves along a geometrical path 20 which is concentric withthe inner and the outer drum. From the following it will be evidentthat--if the diameter of the inner drum 15 is twice as great as thediameter of the cable guide roller 7--for a 400° rotation of the X-raysource 4 and the X-ray detector 5 about the axis 7 it suffices that theouter drum 16 constitutes half a cylinder (drum) and the inner drumthree-quarter of a cylinder (drum).

The cable guider roller 17 is coated with an elastic layer 21, so thatmanufacturing tolerances in respect of the diameter of the inner drum15, the outer drum 16 and the cable guide roller 17 may be comparativelylarge. The inner drum 15 and the outer drum 16 are provided withcircular flanges 22 and 23 respectively, between which the cable guideroller 17 is guided, so that the tolerances during alignment of thecylinder axes of the inner and the outer drum relative to each other maybe comparatively large. The flexible cable 12 is retained between theouter surface of the inner drum 15, the outer surface of the cable guideroller 17 which is provided with an elastic coating 21, and the innersurface of the outer drum 16. It has been found that this method ofretaining the cable has a stabilizing effect on the operation of thecable guide. It has appeared that in the case that the cable guideroller--owing to whatever cause--is not disposed entirely within thecable loop a so-called "creeping-effect" occurs during the movement ofthe cable guide roller, so that the roller has always returned into thecable loop, after some time. The suspected cause of said"creeping-effect" will be further discussed on the basis of angularrotation equations and with reference to FIG. 4.

The X-ray source 4 and the X-ray detector 5 are secured to a frame 24which by means of slides 25 is movable in guides 26 which are mounted onthe inner drum 15. The slides 25--which may for example be driven by anelectric motor in a manner known per se, are movable in a direction 27,perpendicular to the axis 7. By moving the frame 24 relative to thepatient 1, the size of the patient area to be examines can be changed(geometrical magnification). When all detection elements 6 then remainin use, the accuracy with which the density distribution can becalculated will increase when the examined patient area is reduced andwill decrease when said area is enlarged. This freedom of changing theaccuracy of the density distribution calculation at option is of greatsignificance in computer tomography.

FIG. 3 shows a flexible cable 12, which contains ten adjacent cores 31.The cores 31 are retained--viewed in the longitudinal direction of thecable--by means of regularly spaced clamps 32 which extend transverselyto the longitudinal direction of the cable. The clamps 32 comprise twoparts 33 which are clamped around the cable cores with the aid of screws34. The parts 33 of the clamps 32 are provided with beds 35 which aredimensioned so that the centres of the cores 31--viewed in across-section at the location of the clamps--are disposed on a straightline which is substantially parallel to the axis of rotation 7. Such anarrangement prevents unequal mechanical loading of the different cores.The set of such straignt lines, which is obtained by taking allcross-sections of the cable, constitutes the so-called neutral plane ofmechanical loading of the bent cable. The centre lines of the individualcores are disposed substantially in this curved plane. In order toexplain the operation of the cable guide angular rotation equations willbe derived hereinafter (see FIG. 4).

The relative angular rotation about the axis 7 of the inner drum 15relative to the outer drum 16 is designated θ₁. In the present examplewith a stationary outer drum 16 θ₁ consequently equals the absoluteangular rotation of the inner drum 15 about the axis 7. The cable guideroller is represented by a circle 42 of a radius r, whilst the innerdrum is represented by a circle 40 of radius R. The radii r and R arerelated to the said neutral plane of the cable 12. The flexible cable 12is retained between the outer surface 44 of the inner drum 15, the outersurface 45 of the cable guide roller 17, and the inner surface 46 of theouter drum 16. In the case of a relative rotation θ₁, the cable guideroller 17 rolls over the outer surface 44 of the inner drum 15 and overthe inner surface 46 of the outer drum 16. The cable 12 which is passedaround the cable guide roller 17 then rolls from the inner drum 15 tothe outer drum 16 or vice versa depending on the direction of rotation.The angular rotation of the centre of the cable guide roller 17 relativeto the axis 7 upon displacement over the geometrical path 20 isdesignated θ₂ and the angular rotation of the cable guide roller aboutits own axis by θ₃. For a rotation θ₁ of the inner drum 15 relative tothe outer drum 16--in the direction indicated in FIG. 4--a length ofcable θ₂. (R+2r) is unrolled from the outer drum 16. Said length isequal to the cable length θ₃.r which is transmitted by the cable guideroller 17 and to the cable length (θ₁ -θ₂).R which is rolled onto theinner drum 15. It follows that:

    θ.sub.2 ·(R+2r)=θ.sub.3 ·r=(θ.sub.1 -θ.sub.2)·R

and ##EQU1## These equations remain valid in the case of rotation in adirection opposite to that indicated in FIG. 4.

From the equation it is evident that a relative rotation θ₁ of 400°--aswas desired for the computer tomography apparatus shown in FIG. 1 andFIG. 2--causes a rotation θ₂ of 133° of the cable guide roller 17 overthe geometrical path 40 if R=2r. For this rotation the inner drum 15should take up a cable length (400-133) 1/2π·R. In the present exampleit therefore suffices that the outer drum 16 has the shape of half acylinder and the inner drum 15 of three quarter of a cylinder.

It is suspected that if the cable guide roller 17 is coated with anelastic layer 21, said layer is depressed by the cable 12 owing to therigidity of the cable, when the direction of rotation θ₁ is opposed tothe direction of rotation indicated in FIG. 4. The cable 12 then extendsas is shown exaggerated by the reference numeral 49. This means that theeffective diameter of the cable guide roller 17 then decreases and thatthe effective diameter of the inner drum 15 increases. If said decreaseor increase is assumed to be Δ, the following equation isvalid--indicated by an accent--for the rotation of the cable guideroller 17: ##EQU2## so that θ₂ '>θ₂.

Thus it is apparent from the formula that in the case of rotation of thedrums 15 and 16 relative to each other in a direction opposite to thedirection indicated in FIG. 4, the cable guide roller 17 rotates alongfaster with the inner drum 15 than in the case of a relative rotation inthe direction indicated in FIG. 4. This would mean that the cable guideroller 17 in fact continually searches for the loop in the cable 12(creeping effect), which has a stabilizing effect on the operation ofthe cable guide.

In a preferred embodiment of a medical apparatus in accordance with theinvention as shown in the FIGS. 1 and 2, use is made of the previouslymentioned fact that the inner drum need only constitute three quarter ofa cylinder and the outer drum half a cylinder. The axis of symmetry ofthe frame 24 passes through the centre of the circular sectors and theX-ray source may thus move into the free area 50 defined by the openingsin the cylinders. These sectors are utilized when the X-ray source 4 ismoved towards the axis 7, for changing the geometrical enlargement.Thus, the previously discussed and highly desirable freedom is obtainedof adapting the accuracy of the density of distribution calculation.

It is to be noted that by the use of a sectoral inner drum and asectoral outer drum, a comparatively flat construction (see FIG. 1) isobtained.

The cable guide in accordance with the invention is by no means limitedto medical apparatus for computer tomography, although it yield specialadvantages in this application. In the commonly known so-called angularmedical stands the cable guide in accordance with the invention alsoyields the advantage of easy integration with the rest of theconstruction, so that the compactness of the assembly is increasedthereby. The use of the cable guide in accordance with the invention isneither limited to medical apparatus. In general, the cable guide may beused in those cases where energy is to be transmitted through cablesbetween relatively rotatable parts. This energy may be electrical,pneumatic as well as hydraulic.

What is claimed is:
 1. Medical apparatus comprising:an X-ray radiationsource which is rotatable relative to a fixed frame; an electrical powersupply; a flexible cable connecting the source to the power supply; aguide for the flexible cable, including a cylindrical inner drum, acylindrical outer drum disposed concentric to and rotatable relative tothe cylindrical inner drum, a rotatable cylindrical cable guide rollerdisposed between the concentrically disposed drums, the axis of theroller being parallel to the axes of the drums, the flexible cablepassing around the roller, the cable being attached near one of its endsto one of the drums, the cable guide roller being retained between anouter surface of the inner drum and an inner surface of the outer drumand being positioned to roll along geometrical paths which areconcentric with the inner drum and the outer drum; an elastic materialwhich is disposed on and at least partially covers the surface of thecable guide roller and which functions to stabilize the operation of theguide; a second movable frame suspended on the inner drum and movable,with respect thereto, in a direction perpendicular to the axis of thedrums, in which frame the radiation source is mounted; a detectorsecured on the movable frame opposite the radiation source, a centralray from the radiation source intersecting the axes of the two drums andbeing parallel to the direction of movement of the frame; wherein eachof the two drums is shaped as a sector of a circular cylinder andincludes a sector-shaped opening, the two openings being angularlyaligned to define a free area, the radiation source being movable intothe free area when the frame is moved relative to the inner drum.